Hi Mahmoud, As I know, in Comsol, you can define modes only with their effective indices. What I would do is the following:

1) I would set an effective indices around that you expect the fundamental mode.

2) I would look for 20 or 30 modes or as many as you need around this effective index.

3) I would check the found modes by the mode solver and I would select those ones you need and register their effective indices (of course you have to do this at that wavelength from that you want to start the dispersion calculations)

4) I would set the mode solver to find only one mode from this step and set the initial effective index of the required mode

5) Make a step in wavelength and recalculate the effective index using the obtained one in the previous wavelength step.

6) Repeat this for all modes you need.

Hope this helps!

Regards,

Zoltan

Thank you Zoltan Varallyay

I try to simulate 2D unit cell with periodic Boundary condition (picture of unit cell is added to the massage)

Is the setup (you have mentioned) for periodic Boundary condition unit cell?

Because I could not fine effective indices in Periodic Condition.

Could you send me a picture from software set up for this problem?

Best Regards,

Mahmoud